docs(paste): add AI preview credit documentation
```markdown
## Summary of the Pull Request
Adds documentation clarifying that the "Show preview" setting for Paste with AI does not consume additional AI credits. The preview displays the same AI response that was already generated from a single API call, cached locally.
## PR Checklist
- [x] Closes: #32950
- [x] **Communication:** I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass - N/A (documentation only)
- [ ] **Localization:** All end-user-facing strings can be localized - N/A (dev docs only)
- [x] **Dev docs:** Added/updated
- [ ] **Documentation updated:** If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys) and link it here: #xxx
## Detailed Description of the Pull Request / Additional comments
This PR addresses the question raised in issue #32950 about whether enabling preview for Paste with AI costs extra AI quota.
Changes to `doc/devdocs/modules/advancedpaste.md`:
- Added new "Paste with AI Preview" section explaining:
- The `ShowCustomPreview` setting behavior
- Confirmation that preview does **not** consume additional AI credits
- The implementation flow showing a single API call with local caching
- Reference to `OptionsViewModel.cs` lines 702-717
- Added settings documentation table for `ShowCustomPreview`
Fixes#32950
## Validation Steps Performed
- Verified documentation renders correctly in Markdown preview
- Confirmed technical accuracy by referencing `OptionsViewModel.cs` implementation
```
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Co-authored-by: yeelam-gordon <yeelam-gordon@users.noreply.github.com>
